Industrial system of voltage rectification by means of rectifying tubes provided with a control grid



y 5, 1953 P M G. TOULON 2,637,837

INDUSTRIAL SYSTEM OF VOLTAGE RECTIFICATION BY MEANS OF RECTIFYING TUBESPROVIDED WITH A CONTROL GRID Filed Aug. 4, 1947 3 Sheets-Sheet 1 INVENT-"0R" P. M .s. TOULON ATTORNEYS May 5, 1953 p. M. G. TOULON 2,637,337

INDUSTRIAL SYSTEM OF VOLTAGE RECTIFICATION BY MEANS OF ECTIFYING TUBESPROVIDED WITH A CONTROL GRID Filed Aug. 4, 1947 3 Sheets-Sheet 2 GRIGATHODE I INVENTOR R MG. TOU LO N 1 A BY /W ATTORNEYS y 5, 1953 P. M. e.TOULON 2,637,837

INDUSTRIAL SYSTEM OF VOLTAGE RECTIFICATION BY MEANS OF RECTIFYING TUBESPROVIDED WITH A CONTROL GRID Filed Aug. 4, 1947 3 Sheets-Sheet 3 FIG.7.

ATTORNEYS Patented May 5, 1953 INDUSTRIAL SYSTEM OF VOLTAGE RECTI-FICATION BY MEANS OF RECTIFYING TUBES PROVIDED WITH A CONTROL GRIDPierre Marie Gabriel Toulon, Paris, France, as-

signor to Products & Licensing Corporation, New York, N. Y., acorporation of Delaware Application August 4, 1947, Serial No. 766,031In France January 17, 1942 Section 1, Public Law 680, August 8,- 1946Patent expires January 17, 1962 I 21 Claims. (01. 321-21)" My inventionrelates to a new process for improving the shape of the rectifiedcurrent sup plied by mercury arc rectifiers provided with a controlgrid, or by any similar industrial rectifiers, and it relates moreparticularly to devices in which it is aimed at controlling in aprogressive manner the voltage between zero and a maximum value.

It is known to supply the grid with an alternating tension having thesame frequency as the voltage applied to the anodes, but suitablyout-ofphase with reference to the said voltage, the result being aprogressive control of the rectified current from zero up to its maximumintensity; but the shape of the current thus obtained is extremelydefective, especially if the number of the'anodes of the rectifiers islow and the phase lag of the grid voltage appreciable. The voltagesupplied by the rectifier to the load circuit, then very abruptly passesfrom an expected zero value to a value which can reach 80% of'thevoltage, for instance, in the case of a 3 phase system, and

still 50% of the voltage in the case of an -hexa-,

phase system.

The voltage surges thus produced in the A. C. network are extremely highin magnitude and the rectified current obtained comprises a. very largeA. C. component, which entails the necessity of a considerablefiltering.

My invention makes it possible to obtain a current which is much moreregular, with no necessity to use many rectifiers; it reduces the amountof the voltage variations; it consists in disposing, on-each one of thewindings of the transformer supplying current to the rectifiers,auxiliary taps carried to increasing voltages. The highest voltage isfirst rectified, and in the tube which carries out that rectificationthe current is pro gressively reduced; then the smaller voltagecorresponding to the next tap is similarly rectified by means of anothertube, loaded progressively in connection with the said tube, and so on.

Another object of my invention is to so connect the improved abovementioned devices as to reduce the number of the rectifying tubes. Tothat end, the tube which has rectified the voltage on one of thetransformer taps (the current is progressively reduced to zero) iscommuted and transferred to a tap of lower voltage, belonging forinstance to another phase; while the following tap of the first phase isconnected to another tube which had reviously supplied another phase andhad become free. The commutation is effected by a set of sectionalswitches controlled in an appropriate manner, preferably by the de viceitself which controls the apparatus used for changing the lag in phaseof the grids.

Another object of my invention consists of a form of realisation of theabove mentioned distributor so that it performs the duty of both thecommutation of the rectifiers and the control of their grid voltage andphase.

For a better understanding of my invention I have illustratedin theappended drawings the characteristic features of its forms ofrealisation, in connection with a description dealing with instances ofapplication which are illustrative, and non-limitative, of its scope.

Fig. 1 illustrates the shape of the current obtained by means of a,thyratron device of the classical type, controlled by changing the haseof the grid. I

Fig. 2 is a diagram of connections made in accordance with my invention,and which in- .cludes a number of rectifying tubes, each one of them socontrolled that the D. .C. obtained can be varied in a very progressiveway, thus avoiding the production of jerks or surges.

Fig.3 shows the shape of the rectified current obtained with the devicerepresented in Fig. 2.

Fig. 4 shows the type of binding post adopted as output terminal, withthe obvious advantages that it simplifies the execution of the wiringand reduces the cost of the insulators.

Fig. 5 gives an instance of an improved wiring diagram, applying to thecase of a voltage doubler, an obvious advantage of which is to reducethe numberof the discharge tubes; a very satisfactory hexaphase systemis thus realised by the use of only 8 tubes. f

Fig. 6 is a perspective view of the general dispositions of an improvedsectional switch designed for automatically connecting up the varioustubes to the successive terminals of the supply transformer.

Fig. 7 is a detail view of the mechanical means adopted for successivelythrowing into service with the desired connections, the sectionalswitches of Fig. 6.

As shown in Fig. i, a polyphase rectifier (hexaphase in general) gives arectified current of a satisfactory shape when the grid voltages are socontrolled that they are in phase with the anode voltages, as is thecase for the curves l, l, I". On the contrary, whenthe phase differenceis made rather large between the grid and the anode voltages. (so as toreduce themean value of the 0 terminal voltage across a resistanceload), a subsuch 3? monies for the D. C. as well as the A. C. circuits;the representative. curve of that current is, as shown in Fig. l, asuccession of straight segments 5 2; 2, 2", and of sinusoidal sectionssuch as i, l, i" What makes the curve of l particularly defective is,above all, the abrupt starting of the outputcurrent 'oftherectiher atpoints 2, 2, 2", withthe resultbthatap preciable voltage differenccsor'surges occur. in: the corresponding voltages.

For eliminating that drawback, according. to Fig. 2, I provide a largenumber of: taps. on the supply transformer, which for instance has a 3phase star-connected secondarywindingwith. its: neutral at point 3. Onbranch '5 are disposed, forinstance, three auxiliary taps 5, 5.,fionthebranch; 'i, the taps 45, 5', ti, and on the thirdibranch' if thetaps t, 5", and 6". Instead of onev rectifier per phase or branch of theabove secondaryof transformer (as would be the usual practise) I makeuse, on each branch, of 52 independent reotifiers, formingthreegroupsa'ndiprovided individual cnne'ctions to the. variousv taps.aan'd.vto the terminals ('outsi'de andn'eutraD of" thextran'sformer, as shownin'Fig.

If the grids of 'each-on'e of those tubes are so disposed that nodifference in phase exists be tween their potentials andtheanodevoltage, out

put currents are onl'ynelivered by: the' rectifiers is, it" and it"sinccthey receive the-maximmn anodic voltage.

If I maintain exactly in" phase'the grids ofthe ubes so, 2!, and 2 2;while bringing-out of phase the grids of the tubes t9, W, "and 1 9 itmay happen that therefrom result'sa progressive -reduction in thevoltage of tii'e 'loadcircuit, but the shape of hte current curve is farmore satisfactory than it would be-ift'ne rectifier-s 201', ZU'Q'ZEWWereomitted.

From the Fig; 3, which 'shows'th'ecurves of cur-- rent corresponding tothe diagram of connections of I-"ig. 2, it is easy to see that theirregularities of the curves of Fig; 1 are corrected to a large extent.The sinusoidal segment "i I corresponds to the tap I! and is all thatremains ofthe'initial sinusoid of Fig. '1, While the vertical segmentIt! shows what remains of the vertical segments '2 which in Fig. lcorrespondedto the abrupt falls of intensity and were the main source ofo'bjec tiona'ble surges.

When, in turn, the tubes IQ, T9119" are put:

out oi. service. and while they tubes 20,, 28.; 28' have their gridsbrought slightly out 'offphase,

the grids which are HU'W'iHD'hBISGI with: th'e'anod'c-rv are the grids21, 2|, and 21"", which", in consequence, deliver the maximum output:current.

The change in phase is progressively applied to the potentialsof-the'grids 2B, 26'', 23', which op-- eration has the effect of"reducing their current; and when they no longer have any anode currentthe same 'dephasing operation is practiced on thegrids2l,2l'and2i".

Besides the resultant curve obtained from the association of tubes shownin Fig. '2, 'the'Fig. 3 shows in dotted lines the individual curveswhich are adding their effects so as to give the improved results shown.

The winding under. consideration may be realised in a very simplemanner, withnonecessity to introduce delicate inside connections intothe transformer as is the case when having to multiply the number of theanodes (for instance in the case of a. star-polygonal diagram ofconnections).

Moreover, the. applications :of binding-posts such as the onerepresented in Fig. 4 results in reducing, the number oftheoutputinsulators to its minimum (3 pieces being suifici'ent for thewhole of the connections). The binding-post under consideration is ofthe oil type: its oil chamber is shown in Hi, its main terminal in i5,and" the intermediate taps of the transformer correspond to.tiie'intermediate terminals of the binding-post, carried by therespective collars I6,

'5- andit set at proper intervals along the porcelain body of thedevice.

The above described apparatus offers the inconvenience of requiring arather large number of tubes. In another form of application of myinvention, the tubes are so connected that it be comes possible to,reduce their number, by keepthemin. service when they deliver a usefulcurrent. a commutating system being added to the installation forconnecting the tubes to the proper terminals of circuits in proportionof the needs.

I'he diagram of'wiring of Fig. dapplicsrtothc case of anhexaphasedistribution, obtained from; an ordinarystar-transformen Thetubes. lfi'and;

' M are connected in series, and the. same connec tion applies to theother couples of tubes: 25-26. 21-28, E i-56. would necessitate threegroups of such tubes, I rna'lre' use of one more group. Theload (or; im-

pedance of charge) is connected to the. terminals S--S' (Fig. 5-). Thecommon. A-K or anode. cathode terminal or each groupnisi connected to asystem of sectional switcl'ies, so adapted as to. make the successiveconnectionsv neededof the.

' tubes to the various terminals of the transformer l, t, 5, t, and '3.The multiple sectional switch J'i so combined as to-connect the coupleof tubes either to-the terminal' or totermina-l 6", or to terminal 5",or lastly to the neutral terminai' 5 of the transformer (two ofthoseterminals corresponding to intermediate tapson two of the phases of thetransformer, the twoothers.

corresponding to the end terminal and the neutral point of the thirdbranch or phase).

Similarly the sectional switch J2 can connect its group of two tubes(2E2=6 to terminals. T, 6", 5 and 8.

The sectional switch J3 can connect the third group of tubes (27-29)tot", 5, t, and ii and the switch J4 can connect its group of two tubesto terminals 6., 5, d? and 8-.

At first the grid potentials inall" the tubes are left in-phase withreference to the anode, so

as to have themaximuzn t'ensionin the utilisas tioncircuit: thesectional'swit'ches l, i, 'i and E are all closed, while all the othersare open. As the group 2938 is at a lower voltage, it has no outputcurrent.

Then the phase of the grid tension of the tubes 23 M. now connected totap J, is so regulated that the group 29-33, connected to tap 5, takesprogressively the place of the preceding'one.

This change is very progressively made, and during all the time the D.C. tension produced decreases regularly.

When the tubes 23-24 have ceased to deliver current, the tap 6connection is out out and 7 takes its place.

The group 23-E 5, now free, is thus commuted and connected to the tap tbelonging to another phase of the transformer. That permits toprogressivcly reduce the current delivered by the group 25--2'6,connected bytheswitch 7" to the, tap l of that last phase of thetransformer,v and to. replaceit progressivelyhy thecommuted-group Whilean ordinary distribution.

for each phase, by commuting the groups of tubes while at the same timecontrolling the phases and the potential of their grids. The sameoperation can then be made for the tubes 25 26, thus allowing to releasethe connection T and to establish the connection 8", and

so on. I

It is easy to understand that such a process allows to;pa ss from themaximum voltage to zero in a very progressive way.

Fig. 6 shows a perspective of a sectional switch system which can beused for automatically making the connections described with referenceto Fig. 5. The porcelain insulators 31 and 38, set on a metallic frame34, carry the sectional switches 36. The blades of the said switches areoperated'by' insulated connecting rods 39, which receive their movementas secondary cranks, from a gear wheel 40.

All wheels such as 40 are geared with respective small pinions 4|, whichare all mounted on a common shaft 35, and the electric motor 34 rotatingthe said shaft has a direction of rotation electrically controlled. Eachone of the pinions 4| can be coupled with the shaft 35' at thedesired'moment, by means of'a cog-wheel.

and pawl system shown in Fig. '7, and by means of a distributor 32, Fig.6, the clutching or looking operations are produced in succession, inthe desired order.

What I claim is:

1. In asystem for progressively varying the output of a rectifier systemsupplied with voltage from a plural phase transformer, each of thephases of said transformer providing a plurality of output terminals atsuccessively lower voltage, and wherein said rectifier system comprisesa plurality of rectifier pairs connected back to back, meanssuccessively reducing the current in dif-' ferent of said pairs to aminimum, and means for connecting each of said pairs in successiontodifferent ones of said phases after the current in said pair has beenreduced to said minimum.

2. In a' s'ystem for progressively varying the output of a rectifiersystem supplied with voltage from a plural phase transformer having aneutral connection common. to said phases, each of'sa'id' phasesproviding a plurality of output terminals at a plurality of differentvoltages, said rectifier system comprising a plurality of gaseousconduction electronic valves having each a control electrode, means forconnecting one of said valves to a terminalof one" of said phases,said'last mentioned terminal having a predetermined voltage, means forconnecting each of the remainder of said plurality of valves to aterminal of a diiferent one of said phases, said last named terminalshaving each a voltage higher than 'said predetermined voltage, meansprogressively.v reducing the output of each of said valves'to'a outputof a rectifier system supplied with voltage from a pluralphasetransformer having a neutral connection common to said phases each ofsaid phases provided with a'plurality of output terminals at a pluralityof different voltages, said rectifier system comprising a plurality ofgaseous conduction electronic valves having each a; control electrode,means for connecting one of said valves to a terminal of one of saidphases, said last mentioned terminal having a predetermined. voltage.means for connecting each of the remainder of said plurality of valvesto a terminal of a'diffen ent one of said phases, said last namedterminals having each a predetermined voltage higher than saidpredetermined voltage, means for reducing the output of one of saidremainder of said plurality of valves progressively to zero andfor thendisconnecting said last mentioned valve, and means for reconnecting saidone of said remainder of said plurality of valves to a terminal of saidpredetermined voltage.

4. In a system for progressively varying the output of arectifier'system supplied with'voltage from a polyphase transformerhavingv aneutral connection common to said phases, each of said phasesproviding a plurality of output terminals at a" plurality of differentvoltages, said'rectifier system comprising a plurality of back-to-backconnected gaseous conduction electronic valves having each a controlelectrode, a load, means for connecting one of said plura'lityof pairsbetween a terminal of one of said phases at a first voltage, and saidload, means for connecting the remainder of said pairs of valves eachbetween a terminal of one of said phases and said load, said lastmentioned terminals being allv of the same voltage, phase shift meansfor applying progressively phase shifted voltage to' said controlelectrodes of said remainder of said pairs of valves to cut off saidremainder of 'said pairs of'valves, and means for cyclically connectingeach pair of valves when cut-off to a succeeding phase of said polyphasetransformer and to successively reduced voltage terminals thereof.

5. In a system for progressively varying the output of a rectifiersystem supplied with voltage from a plural phase transformer having aneutral connection common to said phases, each of said phases providinga plurality of output'terminals at a plurality of different voltages,said rectifier system comprising a plurality of pairs of gaseousconduction electronic valves having each a control electrode, means forconnecting said pairs of gaseous conduction electronic valves, one pairto a terminal of a first voltage of one'of said phases and eachremaining pair to a different terminal of identical voltage of each; ofsaid phases, means successively reducing the'output of each of saidpairs to a minimum, and means switching each of said pairs to adifferent output terminal when the output of said pair reaches saidminimum.

6. In a system for progressively varying the output of a rectifiersystem supplied with voltage from a plural phase transformer having aneutral connection common to said phases, each of said phases providinga plurality of output terminals at a plurality of different voltages,said rectifier system comprising a plurality of gaseous: rectifierpairs, said plurality equal to one more than the number of said phases,means for maintainlng on'e of said pairs connected to a terminal at afirst voltage and the remainder of said pairs each to a separateone ofsaid phases, and to terminals of said phases having a voltage higherassassin 8 of. phases, means for,g;rad-. g g mespqfzthe gaseousconduce.vv ested-in e me iat terminal said loadrto .cut off, means for cut, offtube intermediate a ter-. 1 1 phase ne l and for ad usting firs the eofto, conductive condition, andrathereatter' r du y varying firinthcigaseousconduction device connected. ed'atet nminal of phase n-1 andsaid 2 ph ses is i s a, ea h of. s idv unhas re ovidedwitht i mina d 1.to n M1 g s ous: ends my. i s-leach f s idea esus somet e m having meansfor gradually 1 varying its GQndQC-ra oad. d m nstingcu forinterconnectingeach of sai aseousconclucload said; lastrnentionedgaseous conducn. m ns etivel betwe n iif rsntiv n i tion device is cut;n,

be: d er ne, at each of a lase andsaai l4... Ln cQmbi-n on; mod fier-syte c p i i ins-a load, nzzt pha ss u-rce of al er a ing cu 1e; eaehphesec m risin a lurali of voltage terminals of difierent voltage, n+1gaseous conduction;devicmeoupled toterminals of said source, eacho said.devices ving a control grid,

t ,o.'..sasequ nd lnversen allel connected palte gav conduction val es-Th omb l ifi q in QQQQFQ QQB with mm means for energizing saidgrids inindividually 8 he si ac f a edct miuetwn m n a): controllable phase,said last named means sucincludes a gaseous co du n l icessiuely:reducing the=outputof each of said de-.

10- A; syst m or rectify n a tsme i-ne ql vices to en-minimum, and meanssuccess v y 0011- age su nlis tQ a load, eomprisin z, 11 alias f: neetinthe gaseaus. con u tion device at said a source of alternating, VQl aQz, W136; s; an. mini-mum outputintermediate a difierent termi nte r meas: r v ding or c i said phases :15 nal efsai'dvsource-and said lead.

at least two voltage termina s of a; fi i ghe 15. In a system forgradually varying flow of nd a e o d erip determined olt gfi a; 1?-rectifiedcurrent between a nolyphase source and arate first grid 0011 11011 6 s u onduction 18- a load, a separate first gaseous conductiondevice conn cted interm diate each oi said te vice; connectedintermediate each phase of said minals- Qf highe pre e ermined t g a dsaid, 3 source-andsaid load, said devicesall connected load, a fur hergri controll d g e lSt GQIi Q- to a point of. substantially identicalvoltage of tion d vice: con ecte etween he ter ina o said-phases,afurther-gaseous.conduction device sec nd lower p ed term n d volta Ofth h connected: intermediate'one phase and said load P e and saidl dandmean r radually at a pointof reduced voltage of said phase, means diningto Zero, current ,fi W :1 thofirst grid 00.11- 310' for reducing to.zero current, flow in the first gas-1 rolled a us: c nduc io de ice.conn c e im eonsconduction device connected to said one termediat the21131 phdfifi; nd sa d oad. and a. phase, means. forthereafte-rconnecting said last switsihiolither aiterdisconnet' gzsai lastmmentioned gaseous conduction device intermetione v 'it 2 .1 5;QQIldliQii Qn. .d lHQQ ifmm Sa l diate a further phase and said load ata point of source, ii. reduced voltage and in conductivecondition, and

11. The'combina-tionin accordancewithclaim means for thereafter reducingto zero current 10 wher in said aseous; con uction deV BB flow in thefirst gaseous conduction device coninverse' parallel connected gaseousconduction nected intermediate: said further phase and said valves.load.

12. In a current control Sy in O v0f e5. 16". A system for rectifyingalternating voltalternating current having a plurality ofvphases, age,comprising, a source of alternating voltage each of said phases having aplurality of output havingphases 1,2,.v each of said phases havterminalsof different. o a a a plurality ing a first terminalat a firstpredetermined volte of gaseous conduct on. dBViceS 1 1 6 011%- ageand asecond terminal at-a second predeterat than the number o phases in aidhill? mined voltage, a load, n first gaseous conduction y Q ph s, Q i0 7continuously limll$- devices each connected -inter-mediate a separatefel'liillg curr nt. from all but one; of sa phas s oneofsaid firstterminals and said" load, one secs W id lflad Viaflnlye-sa e u ond tie 6and gaseous conduction device connected intervice, and from s O 'sa a sVia two of mediate .thesecond terminalofphase n and said ai as uconduction ev es, m s v grade 55. load, means for graduallyreducingcurrent flow tallyv reduclngto zero .cu ntfl w 0 i intermediatethe first gaseous conduction device las 'm l i ned gaseous Conduction dc nd connected to the firstiter-minal of phase n to zero meansforthereafter in er nnecting Said 1 8 o while maintainingcurrent flow 'insaid second said last mentioned gas ous cond n devicsfi gaseousconduction device, means for thereafter in conductive conditionintermediate another so, disconnecting id 1 st, me ti d first gaseousthansaid n O a d phases and said lead. conduction device from said firstterminal of 13, In combination, a load, an n phase source phase n andconnecting said last mentioned first f l rn in rr nt h vin ph e 1 to. ni gaseous conduction device to a second terminal elusive, 11 an nt a hof sa d p as s pwof one of the phases 1 to -n1 in conductive con- Videdwith m terminals 1 t inclusive; a d dition and means for thereaftergradually reh 1381111111941 1 0 m Of a v n p fi. 1' i iduc-ingto zerocurrent flow in the first gaseous ferent predetermined voltage, n+1. gaous 1. conduetion device connected intermediate the ducti n de s 1 t n+1, nc1u iv a h v n first terminal of said last mentioned phase and a ctr l el tr ea s, mea s. for apply ng: said load.

control voltage to said c ntrol ele trod means 17. In a rectifiersystem, a multiphase source 0 in vidual y con r ling firin ti -mes 0fSaid alternating voltage, aloe-d, least one ga es ga eous QQIlQHQii0.11sconductionrectifier .deviceconnected inter: ing each 95. Q. a smediate leach phasaiei said multiphase source intermediate a d1, t, erm,nel d and said load, means comprising an auxiliary oad. an v t e :51gaseous s9, duet gaseousopnduction devicecoupledtdone of said phases forvarying current flow between said phase of said multiphase source andsaid load in a plurality of discrete time separated steps, and means forvarying the current flow in each of said discrete steps from all saidphases in time succession.

18. In a system for controllably rectifying alternating current, a load,a source of multiphase alternating voltage, of n phases, n+1 gaseousconduction devices connected respectively one between each phase of saidsource and said load, and the remaining one connected between one ofsaid phases and said load, means for reducing current flow between saidone of said phases only and said load by controlling one of said gaseousconduction devices, means for thereafter interchanging said gaseousconduction devices to provide two gaseous conduction devices betweenanother of said phases and said load, and means for reducing currentflow between said another of said phases and said load by controlling atleast one of said last mentioned two gaseous conduction devices.

19. In a system for controllably rectifying multiphase alternatingcurrent, a load, a multiphase source of alternating current, at leastone gaseous conduction device interposed between each phase of saidmultiphase source and said load, means for controlling each of saidgaseous conduction devices to transfer current of substantially the samefirst predetermined magnitude, means for varying current transferthrough said gaseous conduction devices in sequence from saidpredetermined magnitude to a second predetermined magnitude, andthereafter from said second predetermined magnitude in sequence to athird predetermined magnitude.

20, In a system for controllably rectifying multiphase alternatingcurrent by means of a plurality of gaseous conduction devices, a sourceof multiphase alternating current, a load circuit, at least one gaseousconduction device interposed between each of the phases of said sourceof multiphase alternating current and said load, a further conductiondevice between one of said phases and said load, and means for effectinga step-by-step control of the flow of current in said phases comprisingmeans reducing the current flow in said further conduction device, andmeans then coupling said further conduction device to a different one ofsaid phases 21. A system for progressively varying the output of arectifier system comprising a transformer having a plurality of outputterminals each at a different voltage of identical phase, said pluralitycomprising at least three, a plurality of gaseous rectifier devices eachhaving a control electrode, switching means periodically coupling saidrectifier devices to different of said terminals in rectifying relationwith each of said terminals, means for applying to the control electrodeof one only of said devices connected to the output terminal of highestvoltage a phase shiftable control potential to vary the output of saidone of said devices, means for progressively shifting the phase of saidphase shiftable control potential, means for applying to the controlelectrodes of the remainder of said plurality of devices controlpotentials adapted to establish outputs therefrom, and means formaintaining constant said last mentioned control potentials during saidprogressive shifting.

PIERRE MARIE GABRIEL TOULON.

References Cited in the file of this patent UNITED STATES PATENTSFOREIGN PATENTS Country Date France Mar. 18, 1935 Number Number

